Today, over-the-air communications channels are being reallocated to higher and higher frequency bands due to increased demand on presently allocated frequency bands in the electromagnetic spectrum. This is largely due to the recent proliferation of communications involving cellular bands, personal communication networks (PCNs) and wireless communications. With this explosive increase in over-the-air communications and the reallocation of communications channels to higher frequencies, there is a great need to provide improved electronic circuits capable of operating at higher frequencies.
One type of electronic circuit used extensively in communications is known as a modulator. Generally, a modulator is used to frequency shift a slow varying or lower frequency input signal, such as one's voice or video signals, up to a higher frequency for transmission. This frequency shifting is accomplished generally by mixing a high frequency local oscillator (LO) or carrier signal with an intermediate frequency (IF) or modulation signal, to produce a modulated output signal having a frequency typically at the sum and the difference of the carrier signal frequency and modulation signal frequency, i.e., a frequency shifted double sideband signal centered around the carrier frequency. One type of widely-used modulator, known as a single sideband (SSB) modulator, can produce either a lower or upper sideband frequency signal by cancelling one of the sidebands at the output of the modulator and thereby reduce the band width occupied by the resultant modulated signal.
Many modulators in use today are quite expensive when used for high frequency applications for a number of reasons. One significant reason is the dramatic increase in costs associated with providing the electronic components designed for processing signals at higher frequencies. In fact, doubling the frequency of the carrier signal will usually more than double the cost of one or more of the modulator components. For example, components such as power dividers, which are typically used in modulators, become increasingly expensive when constructed to process signals at higher frequencies. The increased costs in the circuitry needed for processing high frequency signals is typically passed on to the consumer, resulting in higher prices for communications equipment used for high frequency communications.
Another reason why modulators designed for use at higher frequencies are expensive is due to the difficulty in achieving proper amplitude and phase balance of signals within the modulator circuit. It is often necessary to adjust the phase of signals within the modulator circuit by, for example, introducing additional components. This leads to an increase in the cost of the modulator due to the costs of additional circuit elements and labor associated with correcting and balancing the phase and amplitude characteristics of the modulator circuit. Modulators requiring a high frequency carrier signal input also require generation of a high frequency oscillator or carrier signal. Thus, the cost of the modulator greatly increases due to the increased cost of high frequency oscillators and synthesizers which are needed to provide the requisite high frequency carrier signal.
Many modulators today employ double balanced mixers for mixing a carrier frequency signal with a modulation frequency signal to produce the modulated output signal. However, a problem with these modulators that use double balanced mixers is that in order to generate a high frequency modulated output signal, the carrier signal or local oscillator must initially provide the high frequency signal to the mixer. However, once a high frequency carrier signal is initially introduced to the modulator, it then becomes increasingly difficult to control the modulator's performance due to the fact that stray elements such as capacitance and lead inductance will affect the tuning.
Another problem with modulators using double balanced mixers is achieving sufficient carrier suppression in the output signal. Because the carrier frequency signal is typically not desired in the output signal, any presence of the carrier frequency must be eliminated or suppressed from the output signal, a process which is difficult and expensive to achieve since the output signal is usually quite close in frequency to the carrier signal.
An even further problem with modulators using double balanced mixers is achieving sufficient isolation between the carrier signal and the modulated output signal. The least resistance path for the carrier to get into the modulated signal is through the mixers. This translates to the isolation between the local oscillator (LO) port and the modulated signal (RF) port, commonly known as LO/RF isolation, provided all other precautions have been taken to avoid the leakage of the carrier signal into the modulated output port. LO/RF isolation depends on the balance in the transformers and the diode junctions within the mixer circuit.
Therefore, because of the foregoing problems that exist with present modulators, there is a great and longfelt need to provide improved modulator and demodulator circuits to solve the problems associated with the generation and processing of high frequency modulated signals.